1. Field of the Invention
The present invention relates to an image forming and reproducing apparatus and an image transferring method, and more particularly, to an image forming and reproducing apparatus with independently rotated image carriers on which images of different colors are to be formed, as well as to an image transferring technique used in such an image forming and reproducing apparatus.
2. Description of Related Art
Conventionally, color image forming and reproducing apparatuses are known. In such an apparatus, different color images (such as black, yellow, magenta, and cyan images) are formed separately on associated photosensitive drums or image carriers which serve as image carriers and are rotated independently by driving motors and serving as image carriers. The color images formed on the photosensitive drums are transferred onto an intermediate transfer belt.
Conventionally, such an image forming and reproducing apparatus is provided with rotational phase detecting means for detecting rotation phases of the photosensitive drums independently driven by multiple driving motors, together with phase difference calculation means for calculating a rotational phase difference between the reference phase and the detected rotation phase of each photosensitive drum. The rotation phase of each photosensitive drum is corrected based on the associated rotational phase difference. See, for example, JP 2000-250285A (Publication 1).
The image forming and reproducing apparatus disclosed in Publication 1 is designed so as to correct rotational phase differences among multiple photosensitive drums if there are such phase differences detected. This arrangement allows four different color images, e.g., black, yellow, magenta and cyan images, to be superposed on each other without misalignment. However, since the phase difference has to be corrected every time a rotational phase difference is produced, the operationability and the functionality are degraded.
In order to detect rotation phases of multiple photosensitive drums, a reference pattern, also named a registration pattern, has to be formed on each of the photosensitive drums prior to actual image formation to detect misalignment among different color images. The rotational phase difference between photosensitive drums is corrected based on the amount of misalignment among color images. This means that the actual image forming and reproducing operation is delayed every time misalignment correction is carried out.
In general, misalignment of color images formed by different photosensitive drums and superposed on each other to form a combined image is often caused by error in distance between photosensitive drums, as well as in machining and assembling accuracy of components used in the image writing unit.
Once such an error is corrected at the beginning of using the image forming and reproducing apparatus, it does not greatly change until the photosensitive drums and image writing unit are removed for maintenance.
In the conventional image misalignment correction, the registration position is adjusted dot by dot to shift the image position to the correction position in the image transfer unit. Since the image misalignment is corrected on the dot basis, subtle offset of images within a dot cannot be corrected although such minute offset is likely to affect evenness of color and delicate hue.